The present invention relates to a process for preparing methylenedianiline by reacting aniline with formaldehyde in the presence of acid catalysts, the mixtures which can be prepared by this process comprising methylenedianiline, a process for preparing polyisocyanates by phosgenation of these mixtures comprising methylenedianiline, and polyisocyanates obtainable in this manner.
The preparation of methylenedianiline (also termed MDA below), is generally known and is customarily carried out by continuous or batchwise reaction of aniline with formaldehyde in the presence of acid catalysts. In this reaction, whose main product is 4,4xe2x80x2-MDA, the unwanted byproduct N-methyl-MDA is formed to a small extent. This byproduct is disadvantageous, in particular in the subsequent reaction of the MDA with phosgene to prepare methylenebis(phenyl isocyanate), also termed MDI, since the N-methyl-MDA is the precursor compound for chlorinated byproducts in the MDI and chlorine contents in the MDI as low as possible are sought.
To decrease N-methyl-MDA as byproduct in the preparation of MDA, various processes are known.
Thus, U.S. Pat. No. 5,286,760, for continuous MDA preparation, describes partial neutralization of the reaction mixture between the stage of condensation of two molecules of aniline and one molecule of formaldehyde and the subsequent rearrangement of the intermediate aminobenzylamines, abbreviated as ABA, to give MDA. EP-A 451 442 and DD-A 238 042 disclose, for a continuous process, the addition of formaldehyde over a plurality of process stages. Processes for decreasing the byproduct are also known for batchwise processes. DD-A 295 628 describes the addition of formaldehyde in two steps during the condensation stage, in the first addition the main amount of formaldehyde being added at a low temperature and the second addition of the remaining formaldehyde being performed at the same or higher temperature.
A disadvantage in these processes is the insufficient decrease of the N-methyl-MDA content in the product mixture, so that there is still a need for improvement.
Processes for preparing MDI from MDA by phosgenation are generally known.
It is an object of the present invention to develop a process for preparing methylenedianiline by reacting aniline with formalde-hyde in the presence of acid catalysts which minimizes the N-methyl-MDA content as an unwanted byproduct. Such an MDA should be used, in particular, in an improved process for preparing methylenebis(phenyl isocyanate) (MDI), which should make accessible an MDI having improved properties, in particular a low chlorine content and/or a light color, in particular in the crude MDI which, in addition to the monomeric MDI, also comprises polymeric MDI, and/or should be made accessible in the polymeric MDI.
We have found that this object is achieved according to the invention, in a semicontinuous process, by introducing aniline with or without acid catalyst, feeding formaldehyde with or without acid catalyst through a mixing element into a circuit in which aniline with or without acid catalyst and with or without previously added formaldehyde is circulated and, after feeding in at least 50% of the total amount of formaldehyde to be fed in, heating the reaction mixture to a temperature above 75xc2x0 C.
This novel procedure permits a higher content of higher MDA oligomers to be obtained than is possible by a continuous procedure at high molar ratios of aniline to formaldehyde without recycling the MDA. By the process according to the invention, minimizing the content of unwanted byproducts is possible.
The reaction according to the invention of aniline with formaldehyde, preferably in the presence of acid catalysts, is performed according to the invention semicontinuously, i.e. one reaction component, the aniline and preferably the acid catalyst, is introduced and the second reaction component, the formaldehyde with or without acid catalyst, is added to the first reaction component. Preferably, the process according to the invention is carried out in such a manner that aniline and acid catalyst are introduced and formaldehyde is added to this first reaction component. The reaction is customarily carried out at temperatures of from 20 to 150xc2x0 C. Preferably, the process according to the invention is carried out in such a manner that the formaldehyde is added to the reaction mixture in the circuit, i.e. to the aniline, the acid catalyst and to formaldehyde which has possibly been previously added and reaction products, up to an amount of at least 50% of the total amount of formaldehyde to be fed, preferably up to complete addition of all of the formaldehyde, at a reaction mixture temperature in the circuit of from 20 to 75xc2x0 C., preferably from 20 to 60xc2x0 C., particularly preferably from 30 to 40xc2x0 C.
The temperature effects the isomeric distribution of the methylenedianiline in the product. If, preferentially, 2,2xe2x80x2-and/or 2,4xe2x80x2-methylenedianiline are to be prepared, a high temperature may be advantageous. The reaction mixture can be heated by generally customary devices, e.g. by heat exchangers in the pumped circuit or a second pumped circuit and/or via the reactor wall.
The reaction mixture, after feeding into it at least 50% of the total amount of formaldehyde to be fed, is, preferably towards the end of the feed of formaldehyde solution, particularly preferably after the complete addition of the entire amount of formaldehyde to the reaction mixture, heated, preferably for a period of at least 0.2 hours, particularly preferably from 0.2 to 48 hours, in particular from 0.2 to 6 hours, at a temperature of above 75xc2x0 C., preferably above 90xc2x0 C., particularly preferably from 105 to 150xc2x0 C., especially from 110 to 135xc2x0 C. Particularly preferably, after complete addition of the formaldehyde to the reaction mixture, the reaction mixture can be heated for a period of from 0.1 to 120 minutes at a temperature of from 65 to 100xc2x0 C. and then, as described above, at a temperature of above 75xc2x0 C. This heating offers the advantage that the handleability of the reaction mixture is simplified, since the reaction mixture has a lower viscosity at the higher temperature. At the same time, during this heating, unwanted byproducts in the reaction mixture are broken down or rearranged in an ageing phase. The reaction mixture can be aged under these preferred conditions in the apparatus in which the reaction of formaldehyde with aniline was carried out, or else batchwise or continuously in another apparatus into which the reaction mixture can be transferred after complete addition of the formaldehyde. For example, the reaction mixture can be aged in the apparatus in which the formaldehyde solution is fed or was fed. It is also possible to pass the reaction mixture from the apparatus into at least one further reactor, for example a tubular reactor and/or stirred tank, and to perform the ageing in this reactor (these reactors) at a temperature of above 75xc2x0 C. Preferably, the reaction mixture, after complete addition of the formaldehyde, is transferred to another apparatus in which the ageing is completed. Particularly referably, the reaction mixture, after complete addition of the formaldehyde which took place preferably at a temperature of from 20 to 60xc2x0 C., particularly preferably from 30 to 40xc2x0 C., is transferred into a customary storage vessel, heated as described preferably at a temperature of from 65 to 100xc2x0 C. and then heated in conventional reactors, preferably a tubular reactor, as described preferably at a temperature of from 105 to 150xc2x0 C., particularly preferably from 110 to 135xc2x0 C.
The reaction mixture can thus be passed into, for example, tubular reactors, stirred tanks, stirred tank cascades, combinations of stirred tanks and tubular reactors in which the reaction to give MDA can be completed.
The reaction mixture comprising MDA and customarily polymeric MDA can be worked up after the reaction by generally known processes, for example by neutralization, phase separation, distillation and/or chromatographic separation methods, preferably by neutralization, preferably at from 60 to 110xc2x0 C., and removal of water, aniline and possibly other unwanted minor components by distilling these substances.
Preferably, the reaction mixture is neutralized, preferably with aqueous sodium hydroxide solution, for example 50% strength aqueous sodium hydroxide solution, preferably at from 60 to 110xc2x0 C., and the aqueous phase is then removed by phase separation. To remove inorganic impurities, the organic phase can be washed at customarily from 60 to 110xc2x0 C. with water, the aqueous phase can be separated off and then unreacted aniline can be removed from the organic phase, that is to say the MDA, by distillation, preferably at a pressure of from 1050 to 5 mbar and a preferred temperature of from 180 to 240xc2x0 C.
The starting components formaldehyde, aniline and acid catalyst can be used at customary purities, the formaldehyde being able to be in equilibrium with higher molecular weight addition products such as poly(oxymethylene)glycols. The formaldehyde can be used in customary, for example aqueous, solutions having a formaldehyde content of from 10 to 60% by weight, based on the weight of the solution. The formaldehyde can also be fed in the gaseous state. In this case, it is fed as pure gas or as a mixture with inert gas. If required, water can be added separately.
The reaction mixture can be circulated in a suitable apparatus by generally customary devices, for example pumps. The rate at which the reaction mixture is circulated is preferably from 1 to 6 m/sec. The formaldehyde solution can be fed via a reaction mixing pump, such as described in DE-A 4220239 or via a nozzle system, e.g. a ring-gap nozzle, built into the pump circuit. In the case of the reaction mixing pump, the device not only serves for feeding in the formaldehyde and preferably complete mixing, but also for moving the reaction mixture in the apparatus. If a nozzle is used, the reaction mixture can be moved in the apparatus by conventional pumps known in chemistry. The mixing energy dissipated locally during the feed of formaldehyde into the reaction mixture in the mixing zone of the mixing element, i.e. for example the nozzle or the reaction mixing pump, is preferably from 100 to 100,000 W/l. The quantity in the pumped circuit is in a ratio to the quantity of formaldehyde solution fed into the circuit of preferably at least 20:1.
As acid catalyst, use can be made of catalysts generally known for this reaction, for example acids having a pKa less than 1.5, e.g. mineral acids such as phosphoric acid, sulfuric acid and/or hydrochloric acid (HCl); preferably HCl is used. Aniline and the acid catalyst, preferably HCl, are preferably mixed at from 30 to 60xc2x0 C, preferably from 35 to 45xc2x0 C.
The molar ratio of aniline to acid catalyst in the reaction mixture is customarily from 1:0.6 to 1:0.01, preferably from 1:0.3 to 1:0.05. This molar ratio applies in particular to the particularly preferred embodiment in which aniline and acid catalyst are introduced and then formaldehyde and no further acid catalyst is added.
The molar ratio of aniline to the total amount of formaldehyde to be added is customarily from 1.7:1 to 7.2:1, preferably from 1.9:1 to 5.1:1, particularly preferably from 1.9:1 to 3.6:1. The formaldehyde is preferably fed into the circuit through a nozzle or a reaction mixing pump. In order to avoid unwanted parallel reactions leading to byproducts, the formaldehyde is preferably added in such a manner that as rapid and complete mixing as possible takes place with the reaction mixture which is situated in the apparatus. This can be achieved, for example, by generating a turbulent flow in the mixing chamber. In the process according to the invention, preferably in one apparatus, aniline and preferably HCl as acid catalyst are introduced, mixed, circulated, for example by a connected conventional pump, and formaldehyde is added to this reaction mixture, preferably via a reaction mixing pump or nozzle. The formaldehyde can be added in such a manner that constant volumes per unit time are fed into the reaction mixture until there is a suitable molar ratio of aniline to formaldehyde in the reaction mixture. Preferably, the addition is performed in such a manner that, per minute, from 0.05 to 2% of the original volume of the aniline in the apparatus are passed as volume of formaldehyde solution into the reaction mixture. Instead of introducing a constant volume of formaldehyde per unit time, the formaldehyde can be added to the reaction mixture in such a manner that the volume of the formaldehyde added per unit time decreases in accordance with a mathematical function as the addition progresses. Preference is given to an addition rate which is constant, falling linearly, or falling in stages. Furthermore, the formaldehyde can be introduced in pulses into the reaction mixture, in which case a regular or irregular pulse frequency and addition rate can be selected. The total amount of formaldehyde to be introduced should preferably correspond to the molar ratios described at the outset in relation to the amount of aniline. In this batchwise procedure, the reaction mixture is emptied from the apparatus after the desired conversion rate and further worked up if necessary.